Supramolecular Architecture in Langmuir Films of a Luminescent Ionic

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Supramolecular Architecture in Langmuir Films of a Luminescent Ionic Liquid Crystal Ignacio Giner,†,§ Ignacio Gascoń,†,§ Raquel Gimeńez,†,‡ Pilar Cea,†,§ M. Carmen Lo´pez,†,§ and Carlos Lafuente*,† Departamento de Quı´mica Orgańica-Quı´mica Fı´sica, UniVersidad de Zaragoza, 50009 Zaragoza, Spain, Instituto de Ciencia de Materiales de Aragoń, Facultad de Ciencias, UniVersidad de Zaragoza, 50009 Zaragoza, Spain, Instituto de Nanociencia de Aragoń, UniVersidad de Zaragoza, 50009 Zaragoza, Spain ReceiVed: July 29, 2009; ReVised Manuscript ReceiVed: September 11, 2009

A new luminescent ionic liquid crystal (ILC) with a positive charge in the aromatic central core and two hydrophobic alkyl chains at the ends, called Ipz, has been synthesized and its behavior at the air-liquid (NaCl 2 × 10-6 M) interface has been characterized by combination of surface pressure and surface potential versus area per molecule (π-A and ∆V-A) isotherms, Brewster angle microscopy (BAM) and UV-vis reflection spectroscopy. The results indicate the formation of a supramolecular architecture at the air-liquid interface formed by a head-to-head bilayer on top of a flat-core monolayer. Langmuir films were transferred onto solid substrates, and atomic force microscopy (AFM) and X-ray reflection (XRR) have confirmed that well-ordered multilayer films are obtained. Langmuir films supramolecular structure is different from the molecular organization of Ipz in lamellar liquid-crystalline phases. Furthermore, fluorescence emission spectra reveal a blue shift of the emission band of about 18 nm when compared to the solution spectra. Introduction Ionic liquid crystals (ILC) are a class of liquid-crystalline compounds with some significantly different properties from those of conventional liquid crystals due to their ionic character. Ionic conductivity is typical for ILC and the ionic interactions tend to stabilize lamellar mesophases but ILC also display uncommon mesophases such as the nematic columnar phase.1 ILC are promising materials in molecular electronics for the development of ion-conductive materials,2 in the designing of biosensors,3 and also as building blocks to achieve functional molecular materials by ionic self-assembly1 or Langmuir-Blodgett deposition.3,4 The design and preparation of functional materials requires knowledge and control of nanoarchitectures from the very early stages of self-organization.5 The study of alignment of amphiphilic molecules (schematically formed by a polar head and a hydrophobic tail) at the air-water interface is a very useful approach to obtain further insight about the mechanisms and interactions that drive molecular assembly and supramolecular architecture. Amphiphilic molecules are able to form monomolecular films at the air-water interface (Langmuir monolayers). Also, multilayer formation can be observed by compressing a Langmuir monolayer film of an amphiphile beyond its collapse point.6 Most of them form a stable head-to-head bilayer on the top of a monolayer or a head-to-tail bilayer at the air-water interface; also some examples of tail-to-tail interdigitated bilayers have been reported. Ordinary smectic liquid crystals (LC) form monolayers at the air-water interface, as well as multilayers when compressed,7-9 whereas bent-core molecules usually form multilayers.10,11 The pyrazole ring has been widely used as a ligand in organic, organometallic, and bioinorganic chemistry.12,13 In particular, * To whom correspondence should be addressed: e-mail: [email protected], phone: + 34 976 762295, Fax: + 34 976 761202. † Departamento de Quı´mica Orgańica-Quı´mica Fı´sica. ‡ Instituto de Ciencia de Materiales de Aragoń. § Instituto de Nanociencia de Aragoń.

3,5-diarylpyrazoles have been described as versatile molecules for materials chemistry with a nonlinear shape (boomerang shaped or bent-core-like) and a neat dipole moment along their transverse C2 axis. They are able to display liquid-crystalline properties,14 supramolecular helical organizations,15 luminescence,16 and nonlinear optical (NLO) properties.17 Moreover, their synthetic versatility, which allows substitution in all positions of the heterocyclic ring, leads to the preparation of ionic derivatives with a positive charge in the center of the molecule. These organic salts combine in one structure all the above characteristics (nonlinear shape, LC, luminescence) and the potential to control their organization through the preparation of stable Langmuir films. The aim of this work is the characterization of the behavior of a luminescent ionic pyrazole derivative (Ipz) at the air-liquid interface. This compound is an ionic boomerang-shaped compound with a positive charge in the central part of the molecule and two n-decyloxyl chains at the ends, (Scheme 1) with a tendency to form lamellar liquid-crystalline phases, and is luminescent in the visible part of the spectrum. Upon the compression process at the air-liquid interface, a characteristic plateau is obtained in the surface pressure-area (π-A) isotherms. Molecular organization at this interface has been investigated using surface potential-area (∆V-A) measurements, Brewster angle microscopy (BAM), and UV-vis reflection spectroscopy. Our results suggest that a homogeneous monolayer covers the whole aqueous surface before the plateau is reached, whereas a bilayer above the bottom monolayer is formed in the plateau region. Langmuir films before and at the end of the plateau were transferred onto solid substrates and were investigated by means UV-vis and fluorescence spectroscopy, atomic force microscopy (AFM), and X-ray reflectivity (XRR) to elucidate the supramolecular architecture of the films. Experimental Section Materials. Ipz was prepared as follows: In a 50 mL roundbottom flask were mixed 3,5-bis(4-decyloxyphenyl)-1H-pyra-

10.1021/jp907235n CCC: $40.75  2009 American Chemical Society Published on Web 10/06/2009

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SCHEME 1: Molecular Structure (Left) of Ipza

a Schematic representation of Ipz (top right) in a flat extended configuration and schematic top view of Ipz (bottom right) with their lateral chains perpendicular to the flat core (the arrow inside indicates the direction of the net dipole moment).

zole14 (2.82 mmol, 1.5 g), ground dry potassium carbonate (3.38 mmol, 0.467 g), tetrabutylammonium bromide (0.31 mmol, 100 mg), and 1,3-dichloropropane (2 mL). The mixture was stirred and heated at 130 °C for seven days. Afterward, water (30 mL) was added and the compound was extracted with dichloromethane (2 × 20 mL). The organic layer was dried with anhydrous magnesium sulfate, filtered, and evaporated to dryness to yield a white solid in 40% yield (0.420 g). 1H RMN (400 MHz, CDCl3) δ, ppm: 0.88 (t, J ) 7.2 Hz, 6H), 1.25-1.45 (m, 20H), 1.45-1.48 (m, 4H), 1.68-1.78 (m, 4H), 1.80-1.83 (m, 4H), 3.25 (q, J ) 7,1 Hz, 2H), 4.02 (t, J ) 6.5 Hz, 4H), 5.01 (t, J ) 7.1 Hz, 4H), 6.73 (s, 1H), 7.04 (m, 4H), 7.64 (m, 2H). 13 C RMN (100 MHz, CDCl3) δ, ppm: 14.1, 22.7, 25.7, 26.0, 29.1, 29.3, 29.6, 31.9, 49.1, 68.4, 115.6, 117.3, 129.5, 144.3, 161.7. IR (nujol, NaCl) ν, cm-1: 1498 (arC-C), 1240 (C-O). MS (FAB+): 1120 [M+]. Characterization Methods. 1H and 13C NMR spectra were recorded on a Bruker AVANCE-400 spectrometer. Infrared spectra were obtained with a Nicolet Avatar 380 FTIR. Mass spectra were obtained with a VG Autospec EBE with FABLSIMS using 3-nitrobenzyl alcohol as the matrix. Mesomorphic behavior was examined using an Olympus BH-2 polarizing microscope equipped with a Linkam THMS 600 hotstage central processor and a CS196 cooling system. Differential scanning calorimetry (DSC) was carried out using a DSC 2910 and a Q-2000 from TA Instruments, with samples sealed in aluminum pans and with a scanning rate of 10 °C/min under a nitrogen atmosphere. Films Fabrication and Characterization. Two different troughs were used in this work: a Nima Teflon trough with dimensions 720 × 100 mm2 was used to record simultaneously π-A and ∆V-A isotherms and also to carry out UV-vis reflection experiments, whereas a homemade Teflon trough with dimensions of 460 × 210 mm2 has been used to record BAM images and to prepare LB films. Both were housed in a constant temperature (20 ( 1 °C) clean room. Ultrapure Milli-Q water (F ) 18.2 MΩ · cm) has been used to prepare NaCl solutions (2 × 10-6 M) for the aqueous subphase. The surface pressure (π) was measured by a Wilhelmy paper plate pressure sensor. The spreading solutions were prepared in chloroform (>99.9%) that was provided by Panreac. The solution was spread drop-to-drop using a microsyringe held very close to the aqueous surface, and then the solvent was allowed to completely evaporate over a period of at least 15 min before compression of the Langmuir film at a constant

sweeping speed of 0.02 nm2 · molecule-1 · min-1. Each compression isotherm was registered at least three times to ensure the reproducibility of the results so obtained. The ∆V-A measurements were carried out using a Kelvin probe provided by Nanofilm Technologie GmbH, Germany. A mini-Brewster angle microscope (mini-BAM), also from Nanofilm Technologie, was employed for direct visualization of the films at the air-liquid interface, and a commercial UV-vis reflection spectrophotometer was used to obtain the reflection spectra of the Langmuir films upon the compression process. Langmuir-Blodgett films were deposited at a constant surface pressure by the vertical dipping method, and the dipping speed was 0.6 cm · min-1. The solid substrates used to support the LB films were quartz for the UV-vis and fluorescence spectroscopies and fresh and cleaved mica and Si (100) for the AFM and X-ray reflection measurements, respectively. More details concerning the cleaning procedure of the substrates and the preparation conditions have been reported before.18 UV-vis spectra were acquired on a Varian Cary 50 spectrophotometer. The fluorescence spectra were recorded using a Horiba-Jobin-Yvon Fluorolog 3-22 Tau-3 spectrofluorimeter. The AFM investigations were performed by means of a multimode extended microscope with Nanoscope IIIA electronics from Digital Instruments, using the tapping mode. X-ray Reflectivity experiments were performed with a Bruker D8 Advance, with Cu KR radiation (λ ) 1.54 Å). Continuous scans along omega/2theta (2θ - ω) were obtained. The reflected beam intensity was recorded as a function of the wave vector transfer along the substrate normal. The wave vector transfer (Qz) is directly related to the incident angle, Qz ) sin θinc. × 4π/λ. No off-specular/background scattering has been subtracted, and the intensities are given in arbitrary units because the curves have been rescaled. Simulations were carried out using Leptos software suite where a layered sample model was constructed to generate a simulated reflectivity curve, given the initial mass density, thickness, and interface roughness parameters. Initial parameters were refined to minimize the deviation between the experimental and simulated reflectivity curves. The simulated annealing algorithm was used in the trial-and-error process. Results and Discussion Synthesis and Characterization of the Material. Compound Ipz was synthesized by means of a solid-liquid phase transfer reaction from a precursory 1H-pyrazole. The reaction of 3,5-

Luminescent Ionic Liquid Crystal

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Figure 2. π-A (black line) and ∆V-A (blue line) isotherms of Ipz at the air-liquid interface. Inset figure: Young Modulus vs surface pressure.

Figure 1. UV-vis absorption spectrum (left) and fluorescence emission spectra (right, λex ) 260 and 290 nm) of Ipz in chloroform solution.

bis(4-decyloxyphenyl)-1H-pyrazole with dihaloalkanes like dibromomethane and 1,2-dibromoethane yields the dimeric bis(pyrazolylalkanes).19 Contrary to the expected, 1,3-dichloropropane reacts with a unique molecule of 1H-pyrazole to yield a cyclic compound pyrazolo[1,2-a]-4-pyrazolium derivative as the only product. In this case, cyclization is favored by the formation of a stable 5-membered ring. As a consequence, the signals corresponding to the propylene group appear in the 1H NMR spectra as a quintuplet and a triplet at lower fields than the starting material and coupled as confirmed by a COSY NMR experiment. In the mass spectrum, a single peak corresponding to the molecular cation is obtained. Studies of the thermal properties by polarized optical microscopy and DSC reveal that the compound exhibits two lamellar liquid-crystalline phases on cooling the isotropic liquid. In particular, the compound shows a SmA mesophase at 208 °C characterized by a typical baˆtonnet texture. At the same time, a SmB appears with a dendritic texture and remains on cooling up to 35 °C. The compound is readily soluble in organic chlorinated solvents, which facilitates the deposition of the compound onto the aqueous subphase. Optical Properties. The optical properties of Ipz have been investigated by UV-vis and fluorescence spectroscopy in dilute chloroform solutions. Several concentrations (ranging from 1 × 10-6 to 5 × 10-5 M) have been studied to verify that the Lambert-Beer law is followed for solutions of concentration lower than 5 × 10-5 M. According to this study a 5 × 10-5 M solution has been employed to fabricate the Langmuir films. The UV-vis spectrum of Ipz shows a maximum focused at 290 nm and a shoulder at 260 nm approximately. Ipz exhibits an emission band centered at 426 nm (Stokes shift ca. 11 000 cm-1). Moreover the emission band does not change its position when the sample is excited at 260 or 290 nm as can be seen in Figure 1. Langmuir Films. Figure 2 shows representative π-A and ∆V-A isotherms of Ipz simultaneously obtained onto NaCl aqueous subphase. Surface pressure starts to increase at ca. 1.80 nm2 per molecule and rises upon compression until a characteristic plateau is reached when the area per molecule and the

surface pressure are, approximately, 0.90 nm2 and 30 mN · m-1, respectively. At the beginning of the compression ∆V is ca. 1000 mV, which is indicative of the presence of an ionized film at the air-water interface. Surface potential slightly increases during compression until an area of 2.00 nm2 per molecule is reached; then a sharp increment of 270 mV takes place before the surface pressure takes off. An almost constant ∆V increment can be observed between 1.90 and 0.90 nm2 before a value of ca. 1770 mV is attained. Further compression does not modify significantly ∆V value until a drop at ca. 0.35 nm2. This drop should indicate the end of the plateau region, which is more easily detected in the ∆V-A isotherm than in the π-A isotherm. The inset of Figure 2, shows the Young modulus for the Ipz Langmuir film, Ks defined as the inverse of the film compressibility, Cs, and given by:

KS ) CS-1 ) -A

∂π ( ∂A )

T

(1)

The Young modulus has been used in the literature as an indication of the phase state of the monolayer and to more clearly detect phase transitions. The low values of the Young modulus before the plateau indicate that the Ipz isotherm corresponds to a liquid expanded, LE, type. This interpretation is supported by the fact that no domains are visible in the BAM images before the plateau20 (Figure 3). The strong Coulombic repulsion forces between the charged Ipz units may be the main reason for the existence of this LE phase. The KS minimum located at the plateau surface pressure denotes that a phase transition takes place in this region, which could be interpreted either as a collapse of the monolayer leading to multilayer formation or a transition to a more condensed phase. BAM images show that the whole water surface is covered by a homogeneous expanded film before the plateau beginning, when some bright spots start to appear. Upon compression, these spots increase in number and become brighter, covering the entire surface at the end of the plateau. This suggests that a new layer is formed above the monolayer in contact with the aqueous surface as will be discussed afterward. We have estimated the dimensions of Ipz (Scheme 1) by means of the MOPAC200921 software package using the semiempirical methods PM622 and RM1.23 Molecular dimensions are in good agreement with previously reported X-ray structures for similar compounds.14,24 We also have used both

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Figure 3. BAM images of Ipz at the air-liquid interface during the compression process.

semiempirical methods to verify that the neat dipole moment of Ipz is located along their transverse C2 axis as shown in Scheme 1. At the beginning of the plateau, the area per molecule is ca. 0.90 nm2, whereas the aromatic core has an area of ∼0.60 nm2 (see Scheme 1) in a flat configuration and the projection of each of the lateral chains should be ∼0.18 nm2 in the alltrans hydrocarbon chain configuration. This seems to indicate that, in the ionized monolayer, molecules should have their aromatic core almost parallel to the aqueous surface and the lateral chains as far as possible from this surface due to their hydrophobic character. Whereas the area per molecule at the end of the plateau is nearly a third of the initial value; consequently we can conjecture that the upper layer is probably a bilayer. Another important conclusion about the structure of the multilayer can be obtained from the ∆V-A isotherm: the surface potential remains nearly constant in the plateau; this indicates that the molecules on the upper bilayer do not contribute significantly to ∆V value, which is, basically, that of the ionized monolayer. Thus, the molecules inside the bilayer should adopt an antiparallel orientation where their dipole moments are nearly canceled.25 Multilayer formation and supramolecular organization in Langmuir films were investigated in situ by UV-vis reflection spectroscopy through the reflection of unpolarized light under normal incidence. Reflection spectra recorded at areas per molecule bigger than 2.1 nm2 do not follow a systematic evolution, probably due to the presence of uncovered areas of the aqueous surface underneath the fiber optics detector that

produce significant fluctuations in the signal from one measurement to another. For smaller molecular areas, reflection intensity, ∆R, continuously increases under compression as shown in part a of Figure 4. For low values of absorption, ∆R is given to a reasonable approximation by:26

∆R ) BΓforientε√Rs

(2)

where B is a constant, Γ )(1)/(A) is the surface concentration of chromophores at the air-liquid interface, forient is a numerical factor that takes into account the orientation of the transition moment of the molecules at this interface,27,28 ε is the molar absorptivity, and Rs is the reflectivity of the air-liquid interface at normal incidence. To quantify our results, we have plotted the maximum value of reflection (at λ ) 294 nm) ∆Rmax versus molecular density, Γ, in part b of Figure 4. Before the plateau (molecular density less than 1.1 molecules · nm-2), ∆Rmax is directly proportional to molecular density, that is ∆Rmax × A ≈ constant, which indicates that the orientation of the transition dipole moment of the molecules on the bottom monolayer scarcely changes under compression.29 ∆Rmax also rises in the plateau region following a linear relationship with surface concentration, but the slope is reduced by a factor of ca. 6, which reveals that molecular orientation in the upper bilayer is quite different to the orientation on the bottom monolayer. Finally, there is a new slope change when molecular density reaches a value of ca.


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Figure 5. UV-vis absorption spectra (left) of the one-deposition LB films transferred respectively at 15 mN · m-1 (red line) and 33 mN · m-1 (black line) and fluorescence emission spectra (right, λex ) 290 nm) of these (same colors) together with the emission of the Ipz in chloroform solution (dotted blue line).

To complete this study, we have used the lever rule to estimate the fraction of multilayer domains in the plateau region by means of the following general expression:

fraction of multilayer domains (%) )

Figure 4. (a) UV-vis reflection spectra of Ipz, at the air-liquid interface during the compression process. (b) ∆Rmax at 294 nm vs molecular density; before the plateau (squares), during the plateau (solid triangles), and after the plateau (circles) and the corresponding linear regressions (solid lines). (c) ∆Rmax at 294 nm vs fraction of multilayer domains calculated using the lever rule.

2.6 molecules · nm-2, which corresponds to the end of the phase transition as shown in the ∆V-A isotherm. The dipole transition moment of Ipz molecules in the bottom monolayer are almost parallel to the liquid surface, that is almost parallel to the electric field of the incident light (Scheme 1). The small increment of ∆Rmax due to the upper layer points out that the dipole transition moment on the upper bilayer should be notably more perpendicular to the electric field of the incident light (i.e., almost perpendicular to the liquid surface).

(

)

Ab - A × 100 Ab - Ae (3)

being Ab and Ae the area per molecule at the beginning and at the end of the plateau, respectively, and A the area per molecule. In part c of Figure 4, we have plotted ∆Rmax versus the theoretical fraction of multilayer domains. This figure shows that the lever rule is followed, within experimental error, in the plateau region. Langmuir-Blodgett Films. Langmuir films have been transferred onto solid substrates at 15, 25 (before the plateau), and 33 mN · m-1 (near the end of the plateau) and characterized by UV-vis and fluorescence spectroscopies, AFM, and X-ray reflectometry to investigate the molecular organization in mono and multilayer films. Many deposition experiments carried out at 15 and 25 mN · m-1 have shown that only one film can be transferred before the plateau. However, near the end of the plateau (33 mN · m-1) several films can be successfully transferred with a transference ratio close to 1 and the deposition was Y-type. Figure 5 shows the UV-vis absorption of one-deposition LB films transferred at 15 and 33 mN · m-1. UV-vis absorptions are almost in the same relationship that ∆Rmax at the air-liquid interface. Fluorescence emission spectra of the LB films (not at scale for clarity) and the solution spectrum have also been plotted for comparison purposes. A slight blue shift in the emission band of ca. 5 nm is observed for the LB film transferred at 15 mN · m-1 compared to the solution spectrum, whereas a larger shift of ca. 18 nm is observed in the LB film transferred at 33 mN · m-1. This blue shift indicates that the local environment of the chromophores changes from the monolayer to the multilayer film. The local environment becomes more rigid with the increase of the molecular density in the upper layer. This relatively higher rigidity could cause the blue shift of the emission from the excited state in the same manner as rigidochromism.30 The surface morphology of the films was investigated by AFM. Figure 6 shows three representative images of LB films

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Figure 6. 3D and section analysis AFM images of the one-deposition LB films transferred respectively at 15, 25, and 33 mN · m-1.

(only one deposition) transferred respectively at 15, 25, and 33 mN · m-1. The LB film transferred at 15 mN · m-1 show a homogeneous monolayer covering the whole substrate surface and some 3D sharp defects of height ∼3.5 nm above the monolayer. The LB film transferred at 25 mN · m-1 presents a bigger concentration of 3D defects and also big flat circular domains of height ∼3.5 nm above the monolayer. Whereas the LB film transferred at 33 mN · m-1 presents a homogeneous surface with some dendritic shape defects of depth ∼3.5 nm. The presence of 3D defects and domains in the LB films transferred before the plateau is likely related to the reorganization of the molecules in the film toward an energetically more favorable molecular distribution. In contrast, the lack of 3D defects in the LB film transferred at 33 mN · m-1 is indicative

of a more stable arrangement of the molecules. The film thickness were examined by scratching the films with the AFM tip, yielding values of ca. 1.3 nm for the LB film transferred at 25 mN · m-1 and 4.7 nm for the LB film transferred at 33 mN · m-1, which means that the monolayer presents a height of 1.3 nm and the upper bilayer has a height of ca. 3.4 nm (similar to the height of the defects observed in the LB films transferred at 15 and 25 mN · m-1). XRR measurements were performed on the LB films (only one deposition) transferred onto Si (100) substrates at 15 and 33 mN · m-1 to elucidate molecular organization in the LB films. Experimental XRR data obtained for the LB film transferred at 15 mN · m-1 (not shown) can be fitted with a simple monolayer model where the aromatic cores of Ipz molecules lie almost


J. Phys. Chem. C, Vol. 113, No. 43, 2009 18833 TABLE 1: XRR Fitting Parameters of the LB Film Transferred at 33 mN · m-1 (Only One Deposition) onto Si (100)

Figure 7. XRR of the one-deposition LB film transferred at 33 mN · m-1: experimental (red circles) and simulated data (continuous black line).

SCHEME 2: Schematic Representation of the Molecular Organization of the One-Deposition LB Film Transferred at 33 mN · m-1

layer number

density (g · cm-3)

thickness (nm)

roughness (nm)

1 2 3 4

0.55 0.46 1.06 0.51

1.20 1.09 1.26 1.03

0.10 0.09 0.08 0.12

surface. Layer 2 represents the alkyl chains of the first row of molecules in the bilayer; layer 3 corresponds to the head-tohead aromatic cores of the molecules in the bilayer. Finally, layer 4 represents the upper alkyl chains. Individual layer thicknesses derived from the fitting process are shown in Table 1 together with obtained mass densities and layer roughness. The overall film thickness estimated from these XRR data is ca. 4.6 nm, which is in good agreement with AFM tip scratching. Moreover, the thickness of the bilayer obtained by the XRR fitting is approximately 3.4 nm which is in good agreement with the height of the 3D defects observed in the AFM images. Supramolecular structure of Langmuir films is different from the molecular organization of Ipz in lamellar liquid-crystalline phases although it resembles the architecture of columnar Rh31 or Zn16 metallomesogens or dimeric H-bonded pyrazoles14,15,24,32 previously reported. In the Langmuir films, head-to-head interactions should be favored by intercalated Cl- counterions. Our results point out that Langmuir films open very attractive potentials for ionic liquid crystals to study their molecular organization, for example incorporating different counterions in the subphase, and also to develop new functional materials with desired supramolecular organization and order-dependent response. Conclusions

parallel to the substrate and the lateral chains are nearly perpendicular to the substrate. This model gives a height for the monolayer film of ca. 1.3 nm, which is in good agreement with our previous observations. Taking into account all of the experimental data (experiments at the air-liquid interface and also in LB films), different models have been tested to fit the XRR profile obtained for the LB film transferred at 33 mN · m-1. The better results correspond to the model presented in Scheme 2. We propose that molecules in the bottom monolayer have their aromatic cores almost parallel to the substrate surface, whereas molecules in the upper layer are forming a head-to-head bilayer, with their aromatic cores nearly perpendicular to the substrate surface. Partial interdigitation between the alkyl chains of the bottom monolayer and the upper bilayer has also been considered in this model. In this simplified model, Cl- counterions have not been included although they should be intercalated between aromatic cations to prevent repulsive interactions among them. The XRR curve has been plotted in Figure 7 together with the simulated curve. For modeling purposes, the multilayer film has been divided into four separate layers. Proceeding from the film/substrate interface, layer 1 represents the molecules lying flat over the Si (100) surface with their alkyl chains extended out from the

Stable Langmuir films of a new luminescent ionic liquid crystal, Ipz, have been prepared and characterized onto an aqueous subphase (NaCl 2 × 10-6 M). Langmuir film formation has been investigated using π-A, ∆V-A isotherms, UV-vis reflection and BAM images, showing that a phase transition takes place between 0.90 and 0.35 nm2. Our experiments have shown that a head-to-head bilayer is formed on top of the flatcore monolayer. AFM images and XRR measurements have confirmed the formation of a bilayer about 3.5 nm high. This supramolecular organization is different to molecular alignment in the lamellar liquid-crystalline phases of Ipz and should be favored by Cl- counterions intercalation between boomerangshaped cations. LB films transferred onto solid substrates are fluorescent and a blue shift of ca. 18 nm in the emission band compared to the solution spectra is observed. These results open very attractive perspectives for the development of new functional materials based on ionic liquid crystals. Acknowledgment. We are grateful for the Spanish and European financial assistance (MEC and FEDER) in the framework of the projects CTQ2006-05236, CTQ2009-13024 and MAT2006-13571-CO2-01, and the MICINN CSIC-I3 (project 200860I054). We are also indebted to Aragoń Government for financial support. I. Giner gratefully acknowledges his predoctoral fellowship from the Aragoń Government. The authors are grateful to Luis Morelloń and Laura Casado (Instituto de Nanociencia de Aragoń) for their help in the XRR experiments. We are also grateful to Jordi Dıáz (Universidad de Barcelona) for their efforts in the AFM measurements.

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